Remove unnecessary casts and general code clean-up

pkg/analyzer/lib/source/analysis_options_provider.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.source.analysis_options_provider;
 
 import 'dart:core' hide Resource;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 20 matching lines @@
   /// Provide the options found in [file].
   /// Return an empty options map if the file does not exist.
   Map<String, YamlNode> getOptionsFromFile(File file) {
     var optionsSource = _readAnalysisOptionsFile(file);
     return getOptionsFromString(optionsSource);
   }
 
   /// Provide the options found in [optionsSource].
   /// Return an empty options map if the source is null.
   Map<String, YamlNode> getOptionsFromString(String optionsSource) {
-    var options = <String, YamlNode>{};
+    Map<String, YamlNode> options = <String, YamlNode>{};
     if (optionsSource == null) {
       return options;
     }
 
-    YamlNode doc;
-    try {
-      doc = loadYamlNode(optionsSource);
-    } on YamlException catch (e) {
-      throw new OptionsFormatException(e.message, e.span);
-    } catch (e) {
-      throw new OptionsFormatException('Unable to parse YAML document.');
+    YamlNode safelyLoadYamlNode() {
+      try {
+        return loadYamlNode(optionsSource);
+      } on YamlException catch (e) {
+        throw new OptionsFormatException(e.message, e.span);
+      } catch (e) {
+        throw new OptionsFormatException('Unable to parse YAML document.');
+      }
     }
+    YamlNode doc = safelyLoadYamlNode();
 
     // Empty options.
     if (doc is YamlScalar && doc.value == null) {
       return options;
     }
     if ((doc != null) && (doc is! YamlMap)) {
       throw new OptionsFormatException(
           'Bad options file format (expected map, got ${doc.runtimeType})',
           doc.span);
     }
     if (doc is YamlMap) {
-      (doc as YamlMap).nodes.forEach((k, v) {
+      doc.nodes.forEach((k, YamlNode v) {
         var key;
         if (k is YamlScalar) {
           key = k.value;
         }
         if (key is! String) {
           throw new OptionsFormatException(
               'Bad options file format (expected String scope key, '
               'got ${k.runtimeType})',
               k != null ? k.span : doc.span);
         }
@@ skipping 40 matching lines @@
 /// Thrown on options format exceptions.
 class OptionsFormatException implements Exception {
   final String message;
   final SourceSpan span;
   OptionsFormatException(this.message, [this.span]);
 
   @override
   String toString() =>
       'OptionsFormatException: ${message?.toString()}, ${span?.toString()}';
 }